The present invention relates to an optical module that accommodates an optical system inside a package. For example, the present invention pertains to an optical module, such as a demultiplexer for a wavelength division multiplexing transmission system.
A wavelength division multiplexing transmission technology is a system for multiplexing multichannel optical transmission signals into one optical fiber and is suitable for a high-capacity long-distance transmission system. The optical transmission signal of each channel is, for example, a light beam having different wavelength at a predetermined interval (for example, 0.4 nm) within 1550 nm band. That is, light beams (wavelength λ1 to λn) having different wavelengths, the number of which is n, are used for the optical transmission signals of channels (ch), the number of which is n, that are multiplexed into one optical fiber are.
The wavelength division multiplexing transmission requires light sources for several channels, a multiplexer for combining light signals of several channels into one optical fiber, an optical fiber amplifier, and a demultiplexer for separating the light signals of several channels into each wavelength.
A typical demultiplexer 10 is shown in FIG. 5. The demultiplexer 10 includes a package 11, which accommodates an optical system. The optical system includes a collimator lens 12 and a diffraction grating 13. Optical signals having different wavelengths are multiplexed and transmitted to the package 11 as incident light. The collimator lens 12 converts the incident light into parallel light. The diffraction grating 13 separates the parallel light into each wavelength. A fiber chip 15 for retaining an input optical fiber 14 and a photodetector array 16, which has photodetectors, are secured to the outer surface of the package 11. Output terminals 17 are each connected to one of the photodetectors of the photodetector array 16. A polarized wave compensating filter 18 is accommodated inside the package 11. The polarized wave compensating filter 18 compensates for the direction of travel of each light beam (polarized light) separated into each wavelength by the diffraction grating 13 and returns the light into unpolarized light.
In a module that has an optical system inside the package 11 as the demultiplexer 10, dew formation on the surface of each optical device and deterioration of each optical device must be prevented. For example, if any moisture is included inside the package 11, the moisture forms dew on the surface of the optical device at low temperature, and deteriorates the optical characteristic. Thus, the inside of the package 11 needs to be dried. To prevent due formation during usage or storing under low temperature, the amount of moisture inside the package 11 needs to be less than the moisture calculated from the saturated vapor pressure under that temperature. As shown in FIG. 6, the diffraction grating 13 consists of a retainer 19; a resin molded layer 20, which is formed on the surface of the retainer 19; and an aluminum layer 21, which is formed on the resin molded layer 20 to increase reflection coefficient. If the aluminum layer 21 deteriorates by oxidization, the reflection coefficient of the aluminum layer 21 decreases. Thus, oxidization of the aluminum layer 21 must be prevented.
An inner surface 22 of the package 11 is a surface through which incident light from an input optical fiber 14 passes and also a surface through which light beams separated by the diffraction grating 13 passes toward each photodetector of the photodetector array 16. Therefore, an antireflection dielectric multilayer film is formed on the inner surface 22. Thus, deterioration of the dielectric multilayer film needs to be prevented. The dielectric multilayer film includes three films each formed of magnesium fluoride, silicon dioxide, or titanium oxide. The single-component films are overlapped on the inner surface 22 in this order from the side close to the inner surface 22. Further, a magnesium fluoride film and a lanthanum titanate film (made of composite material of TiO2 and La2O3) are overlapped on the surface of the collimator lens 12 in this order from the side close to the collimator lens 12 to prevent reflection.
To prevent dew formation on the surface of each optical device and deterioration of each optical device, the package 11 may be sealed after being filled with dry nitrogen gas or other inert gas, or the package 11 may be sealed after being decompressed. In this method, gas is filled inside the package, exchanged, and sealed during the manufacturing procedure of the module.
However, hermetically sealing the package increases the restrictions on material and structure, which increases the cost. This is because trapping nitrogen gas inside the package or keeping the inside of the package decompressed requires completely sealed package made of material such as ceramics.
Further, to trap nitrogen gas or other inert gas inside the package, a dedicated device and procedure are required and a procedure for trapping nitrogen gas is required. This decreases working efficiency and complicates the manufacturing procedure, which results in the increase of the manufacturing cost. That is, to perform a procedure for trapping the nitrogen gas under the environment filled with nitrogen gas, a tool such as a glove box filled with nitrogen gas is required. Since nitrogen gas is trapped using such tool, the working efficiency is decreased. Also, when replacing air inside the package with nitrogen gas after assembling the module, a structure that permits gas exchange, such as a hose, needs to be provided on the package. In addition, the replacing procedure needs to be performed. This complicates the manufacturing procedure.
Further, to decompress the inside of the package, a device for decompression such as a glove box and a decompression procedure are required. This decreases the working efficiency and complicates the manufacturing procedure, which results in the increase of the manufacturing cost.